As is well known in the art, the motions of the intake and exhaust valves of an internal combustion engine are timed by the contours of the cams on the camshaft of the engine for opening and closing the intake and exhaust ports of the individual power cylinders of the engine at proper timings to achieve best possible engine performances, particularly, the best volumetric efficiency of the engine. The intake and exhaust valves actuated at the timings thus controlled are concurrently open at least in part at the end of each exhaust stroke and at the beginning of each intake stroke of each of the power cylinders and gives a valve overlap period across the top dead center (TDC) in each cycle of operation of the power cylinder. The valve timings are usually determined with a view to producing maximum intake and exhaust efficiencies and accordingly sufficient amounts of valve overlap when the engine is operating under full power conditions. Under low-to-medium power operating conditions such as, for example, idling conditions of the engine, however, such a valve overlap is excessive for the velocity of the piston movement and, as a consequence, the fresh fuel-air mixture supplied to the combustion chamber of each the power cylinders of the engine tends to blow by into the exhaust port of the cylinder or the exhaust gases to be discharged from the combustion chamber tend to be partially admixed to the fresh fuel-air mixture entering the combustion chamber. This is not only detrimental to the fuel economy of the engine but causes incomplete combustion of the mixture in the combustion chamber and thus gives rise to an increase in the concentration of the toxic unburned compounds in the exhaust gases produced in the engine.
On the other hand, there is an internal combustion engine in which the intake valve is so timed as to remain open until the crankshaft rotation angle of 50 to 60 degrees after the bottom dead center (BDC) on the compression stroke in an attempt to effect inertia supercharging under high speed, full power operating conditions of the engine when increased charging efficiencies are required. In an internal combustion engine of this nature, a problem arises in that the fuel-air mixture which has once been admitted into the combustion chamber is caused to flow backwardly into the intake port under low speed operating conditions of the engine.
With a view to eliminating these problems encountered by valve lifters of the solid type, a hydraulically operated valve lifter has been proposed and put into practice which is capable of continuously varying the opening and closing timings of the intake or exhaust valve in proper relationship to the operating conditions, especially the power output, of an internal combustion engine.
An object of the present invention is to provide a valve lift control apparatus specifically characterized by a hydraulic valve lifter by means of which the opening and closing timings of the intake or exhaust valve to be actuated by the valve lifter and the amount of lift of the valve and accordingly the amount of overlap between the intake and exhaust valves of a power cylinder can be properly varied with the fluid pressure supplied to the valve lifter and accordingly with the varying operating conditions of the engine.
It is another object of the present invention to provide a hydraulic valve lifter characterized in that the movement of the cam for driving the valve lifter is transmitted to the intake or exhaust valve without intervention of any abutting or striking engagement between the movable members included in the valve lifter. More specifically, the driving force imparted to the valve lifter from the cam driven by the engine crankshaft is transmitted through the valve lifter to the push-rod or directly to the rocker arm for the intake or exhaust valve solely by fluid pressures intervening between the individual movable members of the valve lifter so that substantially no mechanical impact occurs in the valve lifter.
The fluid used as the hydraulic pressure medium in a hydraulic valve lifter is, typically, the lubricating oil for the engine. When the engine is being cranked cold during starting or being warmed up after starting, the engine lubricating oil is maintained at low temperatures and tends to be excessively pressurized because of the low fluidity of the oil having a high viscosity at a low temperature. During idling conditions of an internal combustion engine, for example, the pressure of the oil delivered from the engine oil pump increases to the order of 2 to 3 kgs/cm.sup.2 as compared with the pressure of about 1 kg/cm.sup.2 or lower at normal temperatures of the oil. If the oil delivered from an engine oil pump is used for operating a hydraulic valve lifter, therefore, it will happen that the valve timings under cold starting or warming-up conditions of the engine are such that are proper for medium-to-high power conditions of the engine so that the amounts of valve overlap resulting from the valve timings are excessive for, for example, idling conditions of the engine. Furthermore, the high viscosity of the engine oil supplied to the valve lifter impairs the mobility of the movable members in the valve lifter, which therefore behaves as if the same is supplied with a higher fluid pressure than the oil pressure actually supplied thereto. This will cause improper retardation of the valve closing timings and lead to deterioration of the performance efficiency of the engine.
Thus, the present invention further has an object in providing a valve lift control apparatus comprising, in combination with a hydraulic valve lifter a fluid pressure control valve adapted to reduce the fluid pressure to be the valve lifter under predetermined conditions such as cold starting or warming-up conditions of the engine.
Yet, it is another object of the present invention to provide a combination of a hydraulic valve lifter and a fluid pressure control device of the above described nature.